This invention relates generally to fluid dampers for vehicles and more particularly, to dampers of the type which are known as xe2x80x9cactivexe2x80x9d.
The invention is specifically applicable to and will be described with particular reference to a mono-tube shock absorber or strut which is electrically controlled to function with two different damping characteristics. However, those skilled in the art will recognize that the invention is also applicable to twin tube shock absorbers and struts and, in theory, may be applicable to dampers having variable damping rates.
The typical fluid dampers used in vehicle suspensions, such as hydraulic shock absorbers and struts, filter out road inputs from being transferred to the vehicle""s body and associated passenger compartment by dissipating energy. Two common types of vehicle fluid dampers, each having a cylinder and piston, are mono-tube and twin tube shock absorbers. The preferred embodiment of this invention is directed to mono-tube shock absorbers and struts.
As is well known, a mono-tube shock absorber essentially comprises a cylinder or tube filled with hydraulic fluid and into which extends a piston rod having a piston fixed to its end. Generally, the upper end of the piston rod extending out of the tube is adapted for connection to the sprung mass (body) of a motor vehicle and the lower end of the tube or cylinder is connected to the unsprung mass (wheel assembly) of the vehicle. Relative movement of the sprung and unsprung masses of the vehicle produces relative axial movement of the piston which is in sealing sliding engagement with the tube walls and divides the tube into two chambers, conventionally referred to as a rebound chamber on one side of the piston and a compression chamber on the opposite side.
Relative movement of the piston within the cylinder is provided by valving that controls fluid flow from the pressurized chamber past or through the piston to the unpressurized chamber. Two of the more common passive types of valves used in fluid dampers are deflected disc type valves (digressive valves) and blow-off type valves. With a deflected disc valve, a disc stack is positioned as an obstruction in a fluid flow path. During piston movement, once sufficient pressure is developed, the disc stack is deflected to provide an increased flow area. The extent to which the disc stack resists deflection principally determines the damping characteristics of the fluid damper. In a blow-off valve, a single valve disc is generally biased by a spring to normally close-off a fluid flow passage. Sufficient fluid pressure causes the valve to lift, compressing the spring and providing an increased fluid flow area. Different rate springs and preloads allow the valve to blow-off at different pressures thereby regulating damping loads. This invention relates to disc-type valves.
The ride handling characteristics of a damper for a motor vehicle (load vs velocity performance curve) is determined by the rebound and compression characteristics of the piston valve in a passive mono-tube application. However, it is desirable to have at least a two stage damper for both rebound and compression. For example, during vehicle cornering maneuvers in which the piston undergoes low speed compression, it is desirable for the vehicle to have stiff or xe2x80x9cfirmxe2x80x9d ride handling characteristics. Conversely, when the vehicle travels over pot holes at relatively high vehicle speeds in which the piston undergoes high speed compression, it is desirable to have xe2x80x9csoftxe2x80x9d ride handling characteristics. Different vehicles require different handling characteristics. Conventional mono-tube shock absorbers with passive valves can only affect a compromise.
The prior art has long recognized this problem and has developed designs in which the valve orifice, which controls the damping forces, is electrically adjustable. Conceptually, sensors determine the operating condition of the vehicle and algorithms determine a desired orifice size based on the operating conditions. Electronics generate an orifice size signal inputted to electrical apparatus which mechanically adjusts the orifice size. While conceptually sound, there are problems in the commercial implementation of this concept.
The prior art has recognized such problems and has developed a solenoid actuated, shuttle type, shut-off valve. More particularly, an economical solenoid can be designed to fit into a piston rod and develop sufficient force to move a spring biased shuttle valve from an unenergized position to an energized position. A mono-tube piston can be equipped with two passive valves, each controlling rebound and compression, with one of the two valves selectively cut in or out of operation by the solenoid shuttle valve. For example, both valves operate to provide the soft suspension for highway cruising while only one valve operates to provide the stiff compression for cornering. The solenoid is actuated automatically by sensors sensing or predicting the operating conditions of the vehicle. Additionally, the vehicular operator can be provided with a manual override control that forces the solenoid into an energized or de-energized condition. This invention is applicable to this type of active damper and uses a solenoid to selectively cut in and out a passive valve.
Two-stage, solenoid operated active dampers are described in detail in U.S. Pat. No. 5,690,195 to Kruckemeyer et al., issued Nov. 25, 1997 and U.S. Pat. No. 5,706,919 to Kruckemeyer et al., issued Jan. 13, 1998. The ""195 patent illustrates an arrangement where the hydraulic fluid passes in parallel to different valves and the ""919 patent illustrates an arrangement where the hydraulic fluid passes serially through the two valves. This invention is an improvement over the ""919 and ""195 patents, specifically, the ""195 parallel flow patent. The ""195 patent is incorporated by reference herein, specifically for its disclosure of the solenoid, the solenoid actuated shuttle valve and the digressive disc stack valve working in conjunction with the solenoid which are substantially the same as that disclosed herein.
The ""195 patent uses a bi-directional digressive disc stack valve for the passive valve which is always on and typically provides the firm ride handling characteristics of the vehicle. As is well known, bi-directional, digressive disc stack valves cannot provide independently set flow rates for both rebound and compression. Because this valve is normally used to provide the firm handling characteristics of the vehicle, it is highly desirable for the manufacturers, especially those manufacturing xe2x80x9cperformancexe2x80x9d vehicles, to be able to independently set or tune the rebound and compression spring rates of this valve. This is not possible in the ""195 patent.
A more subtle point is that a valve for the firm mode requires relatively high, unimpeded flow rates through the piston. That is, a valve always provides the flow restriction in a passage, i.e., the orifice. However, the passage upstream or downstream of the orifice affects flow through the orifice, i.e., a backpressure at certain flow rates can affect flow through the orifices. Again, the function of the firm valve is to assure high flow rates. Because bi-directional disc stack valves require flow in one direction to unseat the outer edge of the disc and opposite flow to unseat the inner edge of the disc, a serpentine flow path to the orifice often occurs. Such a flow path could adversely affect performance of the valve at certain conditions, i.e., high flow producing turbulence.
Accordingly, it is an important aspect of the invention to provide an improved, parallel flow active damper, especially suited for mono-tube applications, in which at least one of the parallel valves can independently control the flow rate through the damper for both rebound and compression damper modes.
In accordance with one aspect of the invention, a vehicular damper is provided which includes a cylinder and a piston rod in the cylinder having a hollow end defining an axial rod passage and the rod or cylinder moves relative to the other. A cylindrical piston is affixed to the end of the rod and divides the cylinder into a compression chamber and a rebound chamber. The piston has a plurality of firm rebound channels and a plurality of firm compression channels extending therethrough. A uni-directional rebound working disc is in valved contact with the outlet of the rebound channels and a uni-directional compression working disc is in valved contact with the outlet of the compression channels. A valve plate is provided on the rod adjacent the piston and has soft channels extending from an outer end at the rebound chamber to an inner end at the axial rod passage. A solenoid actuated cylinder is movable in the axial rod passage to open and close the inner ends of the soft channels. A bi-directional working disc is provided in valved contact with the soft channels"" outer ends whereby valved parallel flow through the valve plate and piston occurs when the solenoid actuated cylinder opens the soft channels inner ends while the uni-directional discs independently control flow at all times through the firm rebound and compression channels.
In accordance with another aspect of the invention, the piston has a rebound face surface on one side and a compression face surface on the opposite side with a cylindrical rod opening extending therethrough. The rebound face surface has an annular compression valve seat extending therefrom and the compression face surface has an annular rebound valve seat extending therefrom. The rebound disc stack digressive valve includes at least an annular rebound uni-directional working disc and an annular rebound spacer disc with the rebound working disc having an outside diameter greater than the diameter of the rebound valve seat. Similarly, the compression disc stack digressive valve includes at least an annular compression uni-directional working disc and an annular compression spacer disc with the annular compression working disc having an outside diameter greater than the diameter of the compression valve seat. All the discs have substantially circular inside and outside diameters whereby the discs can be mounted at any angular orientation relative to the piston rod for simple assembly. In the preferred embodiment, the digressive valve disc stacks include rebound and compression orifice discs having annular bleed slots extending radially inward from the peripheral edge of the orifice disc. In addition, the disc stacks include annular preload/adjust rebound and compression discs. The orifice disc is between the working disc and valve seat and the preload/adjust disc is between the orifice disc and valve seat. The orifice disc and preload/adjust disc as well as any additional working disc and spacer disc have substantially circular outside and inside diameters void of any aligning notches or protrusions.
In accordance with another aspect of the invention, the piston is an assembly of first and second substantially identical, sintered metal cylindrical halves with each half having on one side a half face surface which is either the piston""s rebound or compression face surface and an interior match face surface on its opposite side with the match surfaces in mating contact with one another to form the piston. Each half has i) on its half face surface, an annular valve seat protruding therefrom which is one of the firm rebound or compression valve seats, ii) a plurality of circumferentially spaced outer channels having outer end openings in the face surface spaced radially outward from the annular valve seat with each outer channel axially tapering in an xe2x80x9cLxe2x80x9d shaped configuration from the outer end face opening to a trapezoidal inner end opening in the match surface (the inner end opening being substantially larger than the outer end opening and radially extending from a position adjacent the rod opening to a position beyond the diameter of the annular valve seat) and iii) a plurality of circumferentially spaced inner channels having face end openings adjacent the half face surface spaced radially inward from the annular valve seat with each inner channel axially tapering in an xe2x80x9cLxe2x80x9d shaped configuration to a trapezoidal match end opening in the match surface. The inner channel match end opening is substantially larger than the inner channel face end opening and (like the outer channel""s inner end opening) radially extends from a position adjacent the rod opening to a position beyond the diameter of the annular valve seat whereby the inner channels of one half mate with the outer channels of the opposite half to form one of the rebound and compression channels while the outer channels of the one half mate with the inner channels of the opposite half to form the other one of the rebound and compression channels. Importantly, the rebound and compression channels have increasing cross-sectional flow areas from the end openings of each channel minimizing any tendency towards turbulent flow or excessive backpressures at high flow rates through the piston. At the same time, the inner and outer channels can be easily formed as straight through passages when the metal (powder metal in the preferred embodiment) is sintered into final half piston form, a material known to those skilled in the art as not conducive to machining operations.
Another aspect of the invention is that an improved damper results with less cost than otherwise required to produce the damper. Specifically, the firm digressive rebound and compression disc stacks allow, as noted, independent control of damper flow in rebound and compression modes to improve damper performance while the rebound and compression disc stacks are free of any aligning notches or protuberances making assembly easier and reducing the time thereof. Additionally, the configuration of the firm rebound and compression channels is relatively easy to form in a sintered part (reducing the cost thereof while enhancing the characteristics of the fluid flow through the channel thereby improving valve performance.
These and other objects, features and advantages of the present invention will become apparent from the following Detailed Description of the Invention taken together with the accompanying drawings.